A good ground plane is beneficial for proper operation of a GPS antenna system. Without such a ground plane, severe multipath effects can disrupt the GPS signal being received by the GPS receiver. In the past, these ground planes have been large and/or bulky, making them very difficult to place inside small areas such as in portable or hand-held GPS systems.
GPS receivers receive signals directly from satellites. However, during use they also may receive indirect "multipath" signals caused by the reflection of the direct signals from large objects such as the earth. In many instances, these indirect signals are phased shifted and act to cancel out and/or distort the direct signals. Such multipath effects are undesirable because they can cause a loss of position data or a decrease in its accuracy.
Although multipath effects can occur over any surface of the earth, it is particularly troublesome over bodies of salt water. Salt water is a relatively good electrical conductor and will therefore reflect GPS signals with little attenuation. Thus, a GPS receiver trying to synchronize to a satellite signal may also receive a signal with similar information and strength from the salt water surface.
To address this and other problems, GPS patch antennas often are used in conjunction with a ground plane. A ground plane serves various functions including isolating the antenna from signals emanating from below them.
Ideally, a ground plane would be an infinite sheet of a perfect conductor. Prior art ground planes have typically employed large geometries to simulate an infinite ground plane. For example, a ground plane used by G. Lachapelle of the University of Calgary and others for the Canadian Hydrographic Service was roughly 1.5 meters in diameter. Others have implemented smaller grounds planes on the order of 50 cm's. These grounds planes are often orders of magnitude too large for use in many portable GPS systems. Another problem with prior art ground planes is that they are often attached to or buried within the ground, limiting their usefulness in portable GPS systems.
Multipath effects may also be reduced electronically through the use of suppression circuits. While they may be smaller than prior art infinite ground planes, the circuitry required to reduce multipath effects is often complex, expensive, and consumes a lot of power. Thus, electronic multipath suppression circuits are often impractical for use in many (especially portable) GPS applications.
U.S. Pat. No. 5,694,136 describes an apparatus and method for using an antenna and a physically small ground plane made from an "R-Card". The "R-Card" ground plane consists of a conductive central region surrounded by a peripheral region have a sheet resistivity that increases as radial distance from the central region increases. This configuration attempts to simulate an infinite ground plane, while remaining smaller than conventional prior art ground planes. However, the "R-Card" ground plane still has a diameter of 13 inches, which is too large for many portable GPS applications. Furthermore the "R-Card" does not suppress surface currents which will adversely affect reception of the GPS satellite data.